Plants

• photosynthesis: the process by which green plants and some other organisms use light energy to make their own food
• carbon dioxide + water --> glucose + oxygen

  6CO2 + 6H2O --> C6H12O6 + 6O2
• temperature:

  with an increase in temperature, the rate of photosynthesis increases. As the reaction is controlled by an enzyme, this trend continues up to a certain temperature until the enzymes begin to denature and the rate of reaction decreases
• light intensity:

  for most plants, the higher the light intensity, the faster the rate of reaction decreases
• carbon dioxide concentration:

  carbon dioxide is also needed to make glucose. As the concentration of carbon dioxide increases, the rate of reaction increases.
• limiting factor: a factor that limits the rate of a reaction, e.g. the amount of light
• Interaction of Limiting Factors:

  • pondweed placed in test tube with water. a capillary tube also containing water leads into the test tube, and it is attached to syringe
  • lamp is placed at measured distance from test tube
  • as it photosynthesises, oxygen is produced, forming a gas tube in the capillary tube
  • the distance the bubble moved is measured using ruler to calculate volume of oxygen
• Light Intensity and Rate of Photosynthesis:

  need a sealed 100ML flask filled with water at room temp, gas syringe, a small amount of pondweed, small lamp and 1M ruler

  • use the ruler to place the flask and pondweed 15CM from the lamp
  • leave the apparatus for 10 mins to allow the pondweed to adjust
  • connect the gas syringe to the flask and record the change in volume on the syringe after 5 mins
  • move the lamp 10CM further away and measure the volume change again. Repeat
• Inverse Square Law:

  light intensity is directly proportional to the rate of photosynthesis because the greater the intensity of light, the more light energy hit the chloroplasts in the leaf, the more photosynthesis can occur at once

  light intensity = 1/distance2
• root hair cells: specialised to take up water by osmosis and mineral ions by active transport from the soil as they are found in the tips of the roots

  • large surface area due to root hairs, meaning more water can move in
  • large permanent vacoule affects the speed of movement of water from the soil to the cell
  • mitochondria to provide energy from respiration for the active transport of mineral ions into the root hair cells
• xylem cells: specialised to transport water and mineral ions up the plant from the roots to the shoots

  • lignin (a chemical) is deposited which causes cells to die. the dead cells become lignified. they become hollow and are joined end-to-end to form a continuous tube so water and mineral ions can move through
  • lignin is deposited in spirals which helps the cells withstand the pressure from the movement of water
• phloem cells: specialised to carry the products of photosynthesis to all parts of the plants

  • cells walls of each cell form structures called sieve plates when they break down, allowing the movement of substances from cell to cell
  • the cells are alive
  • the energy these cells need to be alive is supplied by the mitochondria of the companion cells
  • these cells use this energy to transport sucrose around the plant
• transpiration: loss of water vapour from the leaves and stems of the plant. it's a consequence of gaseous exchange, as the stomata are open so that this occurs

  • water evaporates at the open stomata on the leaf surfaces
  • as water molecules are attracted to each other, when some molecules leave the plants the rest are pulled up through the xylem
  • results in more water being taken up from the soil resulting in a continuous transpiration stream through the plant
• guard cells close and open stomata

  • kidney shaped
  • thin outer walls and thicker inner walls
  • when lots of water is available to the plant, the cells fill and change shape, opening stomata
  • allows gases to be exchanged and more water to leave the plant via evaporation
  • more stomata are found at the bottom of the leaf, allowing gases to be exchanged whilst minimising water loss by evaporation as the lower surface is shaded and cooler
• Translocation: movement of food substances made in the leaves up or down the phloem

  • only occurs in the phloem
  • translocation of sucrose occurs from the sources to the sinks
  • location of the sources and sinks depends on the season
• adaptations of leaves:

  • stomata- able to close to minimise water loss and open to increase evaporation and transpiration. stomata also allow gas exchange to occur when they are open
  • chlorophyll- green which is the most efficient colour for absorbing light. the most possible light is absorbed
  • thinness- leaves are very thin, meaning that carbon dioxide only has a short distance to travel to enter the leaf and oxygen has a short distance to diffuse out
  • large surface area: leaf can absorb more light at once, maximising the rate of photosynthesis
• structure of leaf

  • epidermis is thin and transparent: to allow more light to reach the palisade cells
  • thin cuticle made of wax: to protect the leaf from infection and prevent water loss without blocking out light
  • palisade cell layer at top of leaf: to absorb more light and increase the rate of photosynthesis
  • spongy layer: air spaces allow gases to diffuse through the leaf
  • Palisade cells contain many chloroplasts: to absorb all the available light
• increase in temperature:

  • the molecules move faster, resulting in evaporation happening at a faster rate and therefore the rate of transpiration increases.
  • the rate of photosynthesis increases, meaning more stomata are open for gaseous exchange, so more water evaporates and the rate of transpiration increases
• increase in humidity: if the relative humidity is high, then there will be a reduced concentration gradient between the concentrations of water vapour inside and outside the leaf, resulting in a slower rate of diffusion, which will decrease the rate of transpiration.
• increase in wind: if more air is moving away from the leaf due to it being blown away, then the concentration of water vapor surrounding the leaf will be lower. this will mean there will be a steeper concentration gradient resulting in diffusion happening faster. this will increase the rate of transpiration
• increase in light intensity: this leads to an increased rate of photosynthesis so more stomata open to allow gaseous exchange to occur. this means more water can evaporate leading to an increased rate of transpiration
• gravitropism: the growth of a plant towards a gravity source, such as a light source
• phototropism: the growth of a plant towards light, caused by the plant's response to light
• auxins:

  most plants show positive phototropism because they grow towards the light source

  • the plant is exposed to light on one side
  • auxin, a growth hormone, move to shaded side of the shoot
  • auxin stimulates cells to grow more here
  • this means the shoot bends toward the light
  • the plant receives more light, meaning photosynthesis can occur at faster rate
• most shoots show negative gravitropism as they grow away from gravity. if a shoot is horizontal:

  • auxin moves to the lower side
  • the cells of the shoot grow more on the side with most auxin, so it stimulates cells to grow more here
  • this makes the shoot bend and grow away from the ground
  • this is beneficial as light levels are likely to be higher further away from the ground
• most roots show positive gravitropism as they grow towards gravity. if a root is horizontal:

  • auxin moves to the lower side
  • the cells of the root grow more on the side with less auxin, so it stimulates cells to grow on the upper side
  • this makes the root bend and grow downwards
  • this is beneficial as there are more likely to be increased levels of water and nutrients lower down, and it provides stability for the plant
• auxin as weed killers:

  • many weeds are broad-leaved
  • weedkillers, containing auxin, have been synthesised so they only affect broad-leaved plants
  • the increased amount of auxin causes the cells to grow too rapidly
  • this results in the weed dying
• auxin as rooting powders:

  • plants with desirable features are cloned to make more plants with the same feature
  • one way to clone a plant is to take a cutting from the original plant
  • rooting powder containing auxin is applied to it and it is placed in the ground
  • roots grow and the new plant begins to grow very rapidly
• auxins to promote growth in tissue culture:

  • another way to clone a plant is to use tissue culture
  • cells from the plant are taken are placed in a growth medium containing lots of nutrients
  • hormones such as auxins are added
  • the cells begin to form roots and shoots
• gibberellins are used in germination, for fruit and flower:

  • gibberellins allow seed germination to occur by breaking seed dormancy
  • they allow fruits to grow heavier and larger, increasing yields
  • they encourage flowering plants to flower at a faster rate
• as ethene controls ripening, it's used in the food industry:

  • fruit is picked when it's not ripe
  • it's firm which means that during transport it gets less bruised and damaged
  • when it's needed to be sold, it's exposed to ethene and warmer temperature
  • ethene is involved in controlling cell division and stimulates enzymes that result in fruit ripening
  • this reduces wastage as more fruit is suitable to be sold and it doesn't ripen too early